Genes & Linkage (Edexcel A (SNAB) A Level Biology)

• Every chromosome consists of a long DNA molecule that contains several hundred or even thousands of different genes coding for different proteins
  • A length of DNA that codes for a single polypeptide or protein is called a gene
• The position of a gene on a chromosome is known as its locus (plural: loci)
  • Through experiments and genetic mapping techniques, scientists have been able to work out the specific physical locations of the genes on different chromosomes
  • Each gene occupies a specific locus so that the gene for a particular characteristic is always found at the same position on a particular chromosome
• Each gene can exist in two or more different forms called alleles
• Different alleles of a gene have slightly different nucleotide sequences but they still occupy the same position (locus) on the chromosome

Five different genes found at five different loci

• As its name implies, autosomal linkage only occurs on the autosomes (any chromosome that isn’t a sex chromosome)
• Two or more genes on the same autosome do not assort independently during meiosis
• Instead, these genes are linked and they stay together in the original parental combination
• These linked genes are passed on to offspring all together (through the gametes)

• There are two sex chromosomes: X and Y
• Females have two copies of the X chromosome (XX), whereas males have one X chromosome and one shorter Y chromosome (XY)
• Some genes are only present on one sex chromosome and not the other
• As the inheritance of these genes is dependent on the sex of the individual they are known as sex-linked genes
  • Most often sex-linked genes are found on the longer X chromosome
• If the gene is on the X chromosome, males (XY) will only have one copy of the gene, whereas females (XX) will have two
  • Because males only have one X chromosome, they are much more likely to show sex-linked recessive conditions (such as red-green colour blindness and haemophilia)
  • Females, having two copies of the X chromosome, are likely to inherit one dominant allele that masks the effect of the recessive allele